The emergence of various digital video technologies in industries such as broadcast television, communications networks, consumer electronics, and multimedia computers has been increasing at a remarkable rate. This widespread use of applications is motivated by the fact that signal processing, editing and data transfer of digital information is much easier as compared to performing the same tasks with an analog representation. But more importantly, digital video owes its popularity to the several standards that have been created for digital video compression in recent years.
Digital video compression solutions are arguably the most important component of any digital video platform. Since digital video is known to contain an enormous amount of information in uncompressed format, its manipulation, storage, and transmission can become very time consuming and expensive, if not impossible. As a result, digital video compression techniques are needed to reduce the overwhelming volume of the data while preserving the perceptual quality of its content. A compatible video decompression scheme is then used to uncompress the data for playback.
The MPEG-2 international standard formed by the Moving Pictures and Expert Group, and described in ISO/IEC 13818-2, "Information technology--Generic coding of moving pictures and associated audio information: Video, 1996," which is hereby incorporated herein by reference in its entirety, is intended to satisfy the compression needs of the industries mentioned above. This standard specifies the syntax of the encoded stream and a technique of decoding. It offers great flexibility to the encoder designer allowing the designer to derive his/her own set of compression parameters, and therefore, distinguish his/her product from the rest of the encoders developed by other manufacturers.
The efficiency of the MPEG-2 encoding system is assessed by the fidelity of the perceived video transmitted over a fixed communications bandwidth or stored into a digital medium. For some applications, such as in digital satellite systems, multiple programs are multiplexed into a single large stream for broadcasting, and a bank of MPEG-2 encoders are used to monitor and encode all programs, while maintaining the quality of all received channels. The MPEG-2 stream is sent over either a fixed communications bandwidth or a dynamic bandwidth as in asynchronous transfer mode (ATM) networks. For home consumer video or PC applications, where the size of the storage media is limited, the user would like to accumulate as much information as possible, and an efficient encoder is certainly beneficial in these cases.
Most MPEG-2 encoders are developed to perform in constant bit rate (CBR) mode, where the average rate of the video stream is almost the same from start to finish. A video stream includes a plurality of pictures or frames of various types, such as I, B and P picture types. A picture, depending on its type, may consume more or less bits than the set target rate of the video stream. The CBR rate-control strategy has the responsibility of maintaining a bit ratio between the different picture types of the stream, such that the desired average bit rate is satisfied, and a high quality video sequence is displayed.
Other encoders, including other MPEG-2 encoders, perform in a different mode, defined as a variable bit rate (VBR) mode. Variable bit rate encoding allows each compressed picture to have a different amount of bits based on the complexity of intra and inter-picture characteristics. For example, the encoding of scenes with simple picture content (such as a color test pattern) will consume significantly less bits than scenes with complicated picture content (such as a crowded city street), in order to achieve the same picture quality.
Because of the amount of information that is needed to characterize the video and the complexity of the algorithms needed to interpret the information to effectively enhance the encoding process, VBR encoding is conventionally accomplished in non-real time using two or more passes. In a first pass, encoding is performed and statistics are gathered and analyzed, and in a second pass, the results of the analysis are used to control the encoding process. Although this produces a high quality compressed video stream, it does not allow for real-time operation, nor does it allow for single pass encoding.
Therefore, a need exists for a real-time, single pass variable bit rate encoding scheme that provides high quality videos. A further need exists for a real-time single pass variable bit rate encoding scheme that is easily implementable in hardware.